In a wireless network such as a 3GPP Wideband Code Division Multiple Access (WCDMA) network, system information (SI) distribution provides the ability to schedule and broadcast system information over the network air interface. Typically, system information is generated in the Radio Network Controller (RNC) passed to the Radio Base Station (RBS) using the Node B Application Protocol (NBAP) procedure System Information Update. RBS repeats the information periodically on the Broadcast Control Channel (BCCH) which is mapped to a Broadcast Channel (BCH) transport channel carried by the Primary Common Control Physical Channel (P-CCPCH) in the cell.
System information is grouped into different System Information Blocks (SIBs), where each SIB contains information elements of the same nature. Different system information blocks may have different characteristics, e.g., regarding their repetition rate and the requirements on User Equipment (UEs) to re-read the information.
The Master Information Block (MIB), which is also broadcasted over the air interface, provides references and scheduling information to a number of SIBs in the cell. The scheduling of the MIB is standardized by 3GPP. The repetition period is 80 ms (every fourth 20 ms Transmission Time Interval “TTI”) and the start position is System Frame Number (SFN)=0, i.e., the MIB is transmitted in every BCH TTI starting at SFNs where (SFN mod 8)=0. SIB references can also be provided by separate Scheduling Blocks (SBs). A scheduling block is always referenced from the MIB. FIG. 1 illustrates an example structure of SIBs.
The scheduling information in the MIB/SB provides a list of the SB/SIBs transmitted in the cell and their location on the broadcast channel. The scheduling information also contains a value tag (or an expiration timer) for each SIB that gives information to the UE about the version and validity of the information currently sent on the broadcast channel.
Currently, to acquire the necessary SIBs transmitted on the broadcast channel, the UE must first read the MIB to get references to the first level of SIBs. After that the UE needs to read the Scheduling Block to get references to the remaining SIBs.
One RRC SI message is transmitted in each 20 ms TTI on the BCH using RLC transparent mode. The SI message contains:                The system frame number (SFN) for the first radio frame in the TTI;        a SIB segment and/or one or several complete SIBs (this information is optional and only included if SIB data is scheduled in the TTI).        
If the SI message does not completely fill the transport block, padding is added up to the transport block size of 246 bits.
Different update mechanisms apply for the SIBs depending on whether they contain static or dynamic information. For SIBs containing static information, a value tag is used to indicate when there is a need for the UE to read new information on the broadcast channel. The SIB value tag is sent together with the scheduling information in the MIB or in SB. Whenever the SIB content is modified, the corresponding SIB value tag is updated by the network. Due to the layered structure, a change on the lowest SIB level will propagate all the way up to the MIB, i.e., both the SB- and the MIB value tags will be incremented as well. The new MIB value tag is signaled in message Paging Type 1 (on Paging Channel “PCH”) and System Information Change Indication (on the Forward Access Channel “FACH” and High Speed Downlink Shared Channel “HS-DSCH”) to notify the UEs about the updated system information. Once the UE receives the notification, it will start from the top and compare the new value tags signaled on the broadcast channel with the stored value tags. If they differ, the UE needs to reacquire the SIB to get the updated information.
For SIBs containing dynamic information, an expiration timer is used as an update mechanism. When the timer expires, the corresponding SIB information which the UE has stored is considered to be invalid and the UE must acquire the system information block again.
Note that a particular UE at a certain time may require valid information only for a subset of all SIBs broadcasted in the cell. Which SIBs the UE requires depends on the features supported by the UE and the current RRC state (idle mode, PCH or FACH).
The existing system information distribution mechanism was introduced in 3GPP Rel-99 and has not been changed since then. As a result of the High Speed Packet Access (HSPA) feature growth during the past RAN releases, the available BCH capacity of 12 kbps is almost filled up by existing MIB/SB/SIBs supporting features up to Rel-8.
In particular, the following problems related to the current distribution mechanism have been identified:
Fragmentation:
Although it is possible to concatenate the last segment (or first segment) of a SIB with one or several complete SIBs in the same transport block, the BCH channel is getting more and more fragmented as the number of SIBs and SIB segments increases. The main contributor is the MIB that must be repeated on the broadcast channel every 4th TTI. Even if the MIB itself does not occupy the full transport block, it is difficult to use the remaining parts in an efficient way.
When more features are enabled in the network, the size of the existing SIBs (e.g., SIB3, SIB5 and SIB11) increases and there is a need to schedule more subsequent segments for each SIB. A subsequent segment requires a TTI of its own and cannot be concatenated with other complete SIBs or SIB segments, i.e., it cannot reuse unfilled parts of the transport blocks. Mixing SIBs with different repetition periods also generates “gaps” on the broadcast channel that can be hard to fill considering that the offset between two consecutive segments of the same SIB is limited to 320 ms by the existing standard.
Scheduling Overhead:
The UE is informed about the exact position of each SIB segment as well as the value tag (or expiration timer) and scope (“cell” or “PLMN”) of the SIB. When the number of SIB segments increases on the broadcast channel, the scheduling overhead grow as well. The UE must first read the scheduling information contained in the MIB and the Scheduling Blocks to find the positions of the SIBs to be acquired. Thus, it is important that the scheduling information be repeated frequently to ensure that the overall time to read system information is acceptable. Today, the scheduling information occupies approximately 25% of the total BCH capacity.
Layered MIB/SB/SIB Structure:
The current SIB structure uses up to three layers which can delay the system information acquisition. When a SIB is modified, the UE must first read the MIB, followed by the Scheduling Block, to find out which SIB value tag has been changed. After that, the UE is able to read the new SIB and update the corresponding information.
No DTX Support:
The current SIB structure does not support a DTX format to be used when there is no system information scheduled for the TTI (except the SFN). This costs unnecessary DL transmit power.